I plan to lightly sand the areas and then apply silicon.
Is there anything I have missed?

Rain water is always going to come down the mast. Not sure where your putting silicon. If you mean under the ribber boot that's OK but no need to rough up for silicon to stick.

Silicon is useless as a sealant if it is a keel stepped mast. You need a need a mast boot or use mast boot tape as the deck joint has too much movement for silicon. Have a read of the attached article.
www.cruisingworld.com/how/reader-tip-no-more-leaky-mast/

Water can also come in via halyard exit holes. My boat has the base of the mast in a well with an automatic bulge pump just for the water that comes down the mast.

if you want stop that happening ..... buy a yacht with a deck stepped mast...

There's an episode of RAN Sailing on YT where Johan adjusts a mast drainage hole on his Beason 40 -
Interesting part starts at around 9 minute mark. It may not be relevant to your issue :)

Select to expand quote

crewloose said..
There's an episode of RAN Sailing on YT where Johan adjusts a mast drainage hole on his Beason 40 -

Interesting part starts at around 9 minute mark. It may not be relevant to your issue :)

That was really helpful Crewloose.
It appeared he drilled three holes.
Why wouldn't one be enough?

I thought he drilled a second hole just higher than the original and then pumped in resin to seal the expanding foam (which was a bad choice.)
I guess if you don't use the manufacturer-kit then the option is, after you run a generous diameter conduit (for your cables), you fill the mast at deck level using a non-absorbant foam, maybe you could expand a flat balloon or plastic bag, waxed or PVA'd before injecting resin or similar on top, withdrawing your balloon afterwards. Whether its worth going to all this trouble I don't know.